Abstract

Soccer is an intermittent sport, incorporating low and moderate intensity activity with high intensity actions such as sprinting and acceleration/deceleration (Spencer et al., 2005; Russell et al., 2014). These actions result in fatigue, which is a significant hamstring injury risk factor in soccer (Woods et al., 2004). Hamstring injuries are one of the most common occurring injuries in soccer and typically occur during the latter stages of a match at both 1st team and academy levels (Price et al., 2004; Ekstrand et al., 2011). These injuries are often non-contact in nature, with 80% of these occurring in the bicep femoris (Verral et al., 2003). Injury rates may be reduced by managing training running loads, match minutes and allowing sufficient time for recovery. Measurements of hamstring strength may allow recovery status to be assessed based on running loads. Isokinetic dynamometry (IKD) is the gold standard hamstring strength test (Toonstra & Mattacola, 2013). However, its cost, lack of portability and time taken to test deem it impractical for elite team sports. An isometric test using a handheld dynamometer or sphygmomanometer has been proposed, although these tools do not demonstrate the reliability of IKD (Toonstra & Mattacola, 2013. Alternatively, an isometric test using a portable force platform (FP) has been proposed as an alternative to IKD, demonstrating high reliability in both dominant (CV = 4.3%) and non-dominant (CV = 5.4%) limbs and the ability to detect changes in strength in pre- vs. post exercise tests (McCall et al., 2015). McCall’s group did not assess the validity of force platforms however and suggested further study may be beneficial to assess post-match recovery kinetics in professional soccer. Therefore, the aims of this study aimed to 1) confirm the reliability and validity of FP for hamstring strength testing; and 2) assess hamstring specific recovery via strength testing and examine the relationship between running loads and changes in post-match hamstring strength. In study 1, participants performed 5 isometric knee flexor contractions with both limbs at 90° knee and hip flexion using an FP and IKD. A re-test was performed at the same time of day 1 week later. Force platform reliability was high in the dominant (ICC = 0.95) and non-dominant (ICC = 0.93) limbs. There was moderate correlation between IKD and FP (r = 0.56, moderate) for the dominant limb and high correlation for the non-dominant limb 2 (r = 0.72, strong). However, agreement between IKD and FP measures was generally poor. Despite this, FP’s are still a suitable alternative for hamstring strength testing, provided data is not used interchangeably. In study 2, seven players from an U21 English Premier League (EPL) team were assessed over 3-7 matches (33 observations). Hamstring strength was measured at baseline, +24H and +48H post-match, with GPS used to quantify running loads. Hamstring strength significantly decreased from baseline at +24H and +48H (p ≤ 0.05) in both limbs. At +24H, hamstring strength decreased by 13.6% and 12.5% in the dominant and non-dominant limbs, respectively. At +48H, hamstring strength was still reduced by 9.7% and 10.5% from baseline in both dominant and non-dominant limbs. A significant negative correlation (p ≤ 0.05) was observed between sprint distance and changes in dominant limb hamstring strength. Changes from baseline to +24H demonstrated a moderate negative correlation (r = -0.41), whilst changes from baseline to +48H demonstrated a weak negative correlation (r = -0.39). A relationship between sprint distance and post-match hamstring strength is suggested. Further work is required to identify the causal factors of this reduction in strength. Increases in sprint running loads could further reduce hamstring strength and prolong recovery. Such information may help inform the practitioner’s decisions to individualise training programmes to maximise player availability.